Most digital cameras manufactured for general purpose use include a CMOS image sensor that converts light incident on the camera into electrical signals. CMOS image sensors include an array of pixels positioned at the image plane of the camera. A protective layer, known as a passivation layer, deposited on top of the CMOS image sensor substantially protects the image sensor pixels from damage. The protective layer is typically a transparent nitride or oxide, such as silicon nitride or silicon dioxide.
In certain environmental conditions, including high temperature, poor adhesion and/or stress causes the protective layer to peel off of, or delaminate from, the CMOS image sensor. Process limitations of the photodiodes, microlenses, and color filters in the CMOS image sensor substrate limit available methods of improving layer adhesion that are used for other substrates.
FIG. 1 shows a plan view of a prior-art thin film 100 deposited on a surface 112 of an image sensor 102. In the x and y directions, defined by a coordinate axis 198, the distances between perimeter 101 of thin film 100 and the edges of image sensor 102 are DX and DY, respectively.
FIG. 2 shows a cross-sectional view AA′ of thin film 100 and image sensor 102 of FIG. 1, illustrating peeling 208 from surface 112 of image sensor 102. Peeling 208 may result from a tensile stress 206 within thin film 100 in response to a compressive stress 204 in image sensor 102.
Compressive stresses 204 are caused, for example, by a decrease in ambient temperature. When the thermal expansion coefficient of image sensor 102 is larger than that of thin film 100, image sensor 102 contracts more than thin film 100 in response to a temperature decrease. This unequal thermal contraction results in compressive forces 204 at bottom surface 210 of thin film 100 pointing toward the center of thin film 100. Tensile stresses 206 form within thin film 100 in response to compressive forces 204. Peeling 208 of thin film 100 from surface 112 relieves the interaction of tensile stresses 206 and compressive forces 204.